A power supply circuit that outputs a voltage adjusted at a desired level by switching regulation has been introduced as a power supply circuit for an electronic control device. An example of such a power supply circuit is proposed in JP-A-9-37545 and shown in FIG. 8. A control circuit 105 is connected to a base of an output transistor 101, to which a voltage is applied at its input terminal.
The control circuit 105 produces square-wave duty signals Sd shown in FIG. 9 for switching the output transistor 105, and adjusts a duty ratio in the switching according to a desired output voltage level. An output terminal of the output transistor 101 is connected to a smoothing circuit constructed of a capacitor 102 and a choke coil 107. A diode 106 is connected in parallel with the smoothing circuit.
When the output transistor 101 is turned on by the control circuit 105, a current flows as indicated with an arrow A′ in FIG. 8. As a result, the choke coil 107 is excited and the capacitor 102 is charged. When the output transistor 101 is turned off, energy stored in the choke coil 107 produces a current flow as indicated with an arrow B′ in FIG. 8. In other word, the current flows in a closed circuit containing a freewheel diode 106, the choke coil 107, and the capacitor 102.
The capacitor 102 supplies power supply voltages to loads regardless of the on/off condition of the output transistor 101. Because a charging current is higher than a discharging current when the output transistor 101 is turned on, a terminal voltage Vc of the capacitor 102 increases. The terminal voltage Vc of the capacitor 102 decreases when the output transistor 101 is turned off. The relationship between the terminal voltage Vc and the on/off condition of the output transistor 101 is shown in FIG. 9.
The control circuit 105 monitors the terminal voltage Vc and adjusts the duty ratio of the output transistor 101 so that the terminal voltage Vc remains within a predetermined range. With these operations, a direct-current voltage can be outputted from the power supply circuit at a desired voltage level.
In recent years, electronic control devices for electronically controlling vehicles or home appliances become sophisticated and scales of circuits included in the devices have been increasing. To decrease the size of a substrate, multi-layered substrate is becoming pervasive as a substrate for electronic control devices.
The substrate has an inner layer with common ground wiring pattern for different electronic circuit. Ground terminals of the electronic circuit are connected to the common pattern. When ground terminals of the smoothing circuit and the diode 106 shown in FIG. 8 are connected to the common pattern, a large current repeatedly flows as indicated with the arrow A′ or B′ according to the switching operation.
When the large current flows from the ground terminal of the smoothing circuit and the diode 106, voltages at the connecting points vary due to impedance of the common pattern. If an analog-to-digital (A/D) converter using a ground potential of the common pattern are connected to the ground pattern, high precision analog-to-digital conversion cannot be performed.